8
Journal of Amospherrc and Terreslrial Phvsrcs, Vol. 52, No. 9, pp. 789-796, 1990 Pnnted in Great Bntam. OOZI-9169/90$3.00+ .M) Pergamon Press plc Connections between high- and middle-latitude pulsations J. VERA,* L. HOLLY,* A. ECELAND~ and A. BREKKE~ *Geodetic and Geophysical Research Institute of the Hungarian Academy of Sciences, Sopron, POB 5, H-9401, Hungary : 1_ Physical Institute, University of Oslo, 03 16 Oslo, Norway ; : Aurora1 Observatory. University of Tramso. 9100 Tromsn. Norway (Recriaed in,jinul,fimn IO July 1990) Abstract-Pulsation data from the mid-latitude observatory Nagycenk have been compared with those of the aurora1 zone station Tromsar and of the high-latitude stations Hornsund and Ny Alesund. The comparison has shown a rather high correlation between the Pc3 pulsation activities at all sites, with an independent component at the highest latitudes. Mid-latitude I+3 is also correlated with high-latitude Pc4 and Pc5. The results can be explained by two main types of (mid-latitude) pulsations occurring sim- ultaneously or independently at different times. The first has constant periods up to the highest latitude studied and is thought to be of extramagnetospheric origin. Magnetospheric signals, however, have-at least in certain events-periods increasing with increasing latitude, up to the high-latitude stations. 1. INTRODUCTION High-latitude pulsations are of great importance for the study of different types of geomagnetic pulsations. The problems connected with these pulsations are very complex, however. As sources, both extra- magnetospheric and inframagnetospheric ones are possible (see, for example, the review of YUMOTO, 1986). Extramagnetospheric signals propagate through the magnetopause and the magnetosphere to lower latitudes, essentially conserving their spectrum, but the amplitudes decrease due to damping or energy loss. This damping is expected to be quicker in the case of surface waves at the magnetopause due to the Kelvin-Helmholtz (K-H) instability, and less quick in the case of compressive AlfvCn waves from the upstream ion-cyclotron instability (YUMOTO, 1985). As an inframagnetospheric source, field line reson- ances (SOUTHWOOD, 1974; CHEN and HASEGAWA, 1974) are characterized by periods changing with lati- tude. At low-latitudes, the period increases with increasing latitude; at the plasmapause, there should be a sudden drop in the period ; and outside the period increases again with increasing latitude (ORR and MATTHEW, 1971). The low-latitude part of this increase is experimentally rather well established, while at the plasmapause and at even higher latitudes, the periods can be estimated with greater errors due to a higher level of disturbance. Nevertheless, WEDEKEN (1984) traced between L - 4.5 and 6.5 an increase of the PSC period from 130 to 200 s. FRASER et al. (1988) have shown for an ISEE-orbit that the periods of the field line resonances do not follow the values calculated in the presence of a sharp plasmapause. They attributed the difference to the effect of heavy ions. Thus the period would change, both with and without the plasmapause, quite smoothly toward higher latitudes as observed. A correlation is implied in this supposition between heavy ion concentrations and the presence of the plasmapause. PLYASOVA-BAKOUNINA et al. (1982, 1986) emphas- ized that the two types of pulsations can be identified from records of several stations at different latitudes ; if the periods are equal everywhere, then the event is of solar wind origin, and if they are different, then it is of magnetospheric origin. PLYASOVA-BAKOUNINA et al. (1986) also found that pulsations of the solar wind origin-type have an amplitude maximum at very high latitudes (above 74.), while the maximum of pul- sations of a magnetospheric origin is at much lower latitudes (around 53 ‘). Secondary maxima may exist in both cases, too. It is also possible that, due to longer periods at higher latitudes, the spectral peak ‘moves out’ of the Pc4 range to Pc5, and thus it might remain undetected. In a detailed investigation of some selected events, Cz. MILETITS et al. (1990) found mixtures of two types of pulsations. In a part of the events studied, there were two distinct lines in a plot of period vs latitude: one of them had a constant period, cor- responding to the period expected from the sim- ultaneous IMF scalar magnitude (160/B = T) with B in nT and Tin s, and the other branched off at some latitude around 45 and continued at least to about 55-60”, with increasing periods at higher latitudes. 789

Connections between high- and middle-latitude pulsations

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Journal of Amospherrc and Terreslrial Phvsrcs, Vol. 52, No. 9, pp. 789-796, 1990

Pnnted in Great Bntam. OOZI-9169/90$3.00+ .M)

Pergamon Press plc

Connections between high- and middle-latitude pulsations

J. VERA,* L. HOLLY,* A. ECELAND~ and A. BREKKE~

*Geodetic and Geophysical Research Institute of the Hungarian Academy of Sciences, Sopron, POB 5, H-9401, Hungary : 1_ Physical Institute, University of Oslo, 03 16 Oslo, Norway ;

: Aurora1 Observatory. University of Tramso. 9100 Tromsn. Norway

(Recriaed in,jinul,fimn IO July 1990)

Abstract-Pulsation data from the mid-latitude observatory Nagycenk have been compared with those of the aurora1 zone station Tromsar and of the high-latitude stations Hornsund and Ny Alesund. The comparison has shown a rather high correlation between the Pc3 pulsation activities at all sites, with an independent component at the highest latitudes. Mid-latitude I+3 is also correlated with high-latitude Pc4 and Pc5. The results can be explained by two main types of (mid-latitude) pulsations occurring sim- ultaneously or independently at different times. The first has constant periods up to the highest latitude studied and is thought to be of extramagnetospheric origin. Magnetospheric signals, however, have-at least in certain events-periods increasing with increasing latitude, up to the high-latitude stations.

1. INTRODUCTION

High-latitude pulsations are of great importance for

the study of different types of geomagnetic pulsations. The problems connected with these pulsations are

very complex, however. As sources, both extra- magnetospheric and inframagnetospheric ones are

possible (see, for example, the review of YUMOTO, 1986). Extramagnetospheric signals propagate through the magnetopause and the magnetosphere to lower latitudes, essentially conserving their spectrum,

but the amplitudes decrease due to damping or energy loss. This damping is expected to be quicker in the

case of surface waves at the magnetopause due to the

Kelvin-Helmholtz (K-H) instability, and less quick in the case of compressive AlfvCn waves from the

upstream ion-cyclotron instability (YUMOTO, 1985). As an inframagnetospheric source, field line reson-

ances (SOUTHWOOD, 1974; CHEN and HASEGAWA,

1974) are characterized by periods changing with lati- tude. At low-latitudes, the period increases with increasing latitude; at the plasmapause, there should be a sudden drop in the period ; and outside the period increases again with increasing latitude (ORR and

MATTHEW, 1971). The low-latitude part of this increase is experimentally rather well established, while at the plasmapause and at even higher latitudes, the periods can be estimated with greater errors due to a higher level of disturbance. Nevertheless, WEDEKEN (1984) traced between L - 4.5 and 6.5 an increase of the PSC period from 130 to 200 s.

FRASER et al. (1988) have shown for an ISEE-orbit that the periods of the field line resonances do not

follow the values calculated in the presence of a sharp

plasmapause. They attributed the difference to the effect of heavy ions. Thus the period would change,

both with and without the plasmapause, quite smoothly toward higher latitudes as observed. A correlation is implied in this supposition between

heavy ion concentrations and the presence of the plasmapause.

PLYASOVA-BAKOUNINA et al. (1982, 1986) emphas-

ized that the two types of pulsations can be identified from records of several stations at different latitudes ; if the periods are equal everywhere, then the event is of solar wind origin, and if they are different, then it

is of magnetospheric origin. PLYASOVA-BAKOUNINA et al. (1986) also found that pulsations of the solar wind origin-type have an amplitude maximum at very high latitudes (above 74.), while the maximum of pul-

sations of a magnetospheric origin is at much lower

latitudes (around 53 ‘). Secondary maxima may exist in both cases, too. It is also possible that, due to longer

periods at higher latitudes, the spectral peak ‘moves out’ of the Pc4 range to Pc5, and thus it might remain

undetected. In a detailed investigation of some selected events,

Cz. MILETITS et al. (1990) found mixtures of two types of pulsations. In a part of the events studied, there were two distinct lines in a plot of period vs latitude: one of them had a constant period, cor- responding to the period expected from the sim- ultaneous IMF scalar magnitude (160/B = T) with B in nT and Tin s, and the other branched off at some latitude around 45 and continued at least to about 55-60”, with increasing periods at higher latitudes.

789

790 J. VERB et al.

In a few cases, the increase continued outside the plasmasphere, too.

A comparison of the two observations indicates that the same pulsation event may include both types, those of solar wind origin and of magnetospheric origin, at a changing rate.

The present investigation is aimed at a comparison of mid- and high-latitude pulsations to obtain data about the simultaneous or differing occurrence of pul- sations in the two latitude regions in order to test the

ideas mentioned in this Introduction.

2. OBSERVATIONAL MATERIAL

In the present study, pulsation records of three

Norwegian high-latitude stations were compared to those of the low-latitude station Nagycenk at a similar longitude. The main data of these records are found

in Table 1 (see also EGELAND et al. 1986). In Table 1, A means that hourly average amplitudes were used in

the corresponding category, and 0 means that hourly occurrences are available.

In HSD and TOS, La Cour-type rapid-run mag- netic records were used, in NYA chart records of the

magnetic field with rather low time resolution and, in NCK, rapid-run Earth-current records and data from

the routine processing of the observatory. Amplitudes were in each case handscaled (or the occurrence of a

certain type noted). No effort was made to correct for

the different amplitude characteristics of the recording

instruments (and types of records), as only relative amplitudes were compared without reference to the change of amplitudes vs some other parameter, e.g.

latitude. It should be remarked here that the present results

refer to geomagnetically quiet and moderately dis-

turbed intervals, as intervals with strong activity were excluded from the present study.

3. RESULTS OF THE INVESTIGATION

3.1. Daily variation of amplitudes/occurrences

Daily variations of different types of pulsations carry restricted information about source mechan- isms, as any kind of damping may have a severe influ- ence on them.

The data in Table 2 and Fig. 1 show that the Pc3 peak changes little between latitudes of 47 and 75” : at mid-latitudes there is either a secondary peak around

midday in addition to the main one, or the midday peak is the highest. Pc4 is very similar to Pc3 at very

high-latitude stations ; at lower latitudes and in the

aurora1 zone the daily variation is different+ssen- tially flat with slightly greater amplitudes at dawn and

dusk. Pc5 has a Pc3-like daily variation at NYA ; at HSD, it is of the same type, but the peak appears

much earlier. The shift of the activity maximum at HSD from Pc5 around 6 h LT to Pc3-4 at 9 h LT

may be connected with the change of the toroidal eigenperiod (at 72-), being 5-6 min at 6 h LT and 3 min at 9 h LT (Mead and Fairfield model, mass density of

0.5a.m.u./cm’ at the Equator, at 12 R, and 10 R,, respectively; WARNER and ORR, 1979). SINGER et al. (1981) found a similar shortening of the toroidal eigenperiod in the morning hours for latitudes higher

than about 68’ using the Olson-Pfitzer magneto- spheric model. Thus the shortening of the period at

HSD may be due to this change of the eigenperiod,

while at NYA no similar change was found, perhaps

as near the cleft/cusp position it cannot be observed any more.

Pc5 daily variations at TOS and NCK are more or

less similar to the Pc4 distribution.

Thus the daily variation of the amplitudes/

occurrences of the pulsation types Pc3-5 within the

aurora1 zone show essentially the same charac-

teristics (disregarding the change of the characteristic

period at the edge of the polar cap, for example at

Table 1.

Type of pulsations Station Abbreviation Geomagnetic latitude Interval used and parameter used

Ny Alesund NYA 75.4” 11 September-l 3 October 1984 Pc3p5 A

Hornsund HSD 73.4 8 January-8 February 1985 Pc3 A PC450

Tromso 66.0’ 11 September-l 3 October 1984 8 January 8 February 1985

Pc3 A. Pc2 and Pc4-5 0

Nagycenk NCK 47.0’ 11 September-13 October 1984 8 January-8 February 1985

Pc2Z5 A

Connections between high- and middle-latitude pulsations

Table 2. The daily maxima (LT) of the activity of different pulsations at different stations in the months studied

Station PC3 PC4

NYA g--10 h 8-10 h (essentially similar)

HSD 9h 9h

TOS 8-10 h Slight morning and evening peaks

NCK 10 or 12 h In the late afternoonevening

PC5

8-10 h

6h

14 h and several secondary maxima in the morning and evening

At midday or morning and evening

NYA

300

200

0 HSD 300

& NCK a 300

"0 4 6 12 16 x) 0 0 4 6 12 16 20 0 0 4 8 12 1620 0 0 4 6 121620 OU 0 4 6 12 16 20 0 0 4 6 12 16 20 0 0 4 6 12 16 20 0 0 4 6 12 16 20 0 L

PC2 PC3 PC4 PC5

Fig. 1. Daily variation in the amplitude occurrence frequency of Pc2--5 pulsations at the stations NYA,

0 Sept 1984 x Jan. 1985

791

HSD, TOS and NCK, for September 1984 or for January 1985, or for both (TOS and NCK). For Pc5 in HSD, the daily variation of Pc4 amplitudes is also plotted for comparison at the same station. All values

are reduced to a daily average (= 100).

HSD) ; in the aurora1 zone and at even lower latitudes, 3. The values given in Table 3 are typical of the other only Pc3 has the characteristic peak before local noon. possible pairs, too (e.g. the correlation between HSD P&5 are, at the latter sites, different with a less and NCK is 0.84). The characteristic points in Table pronounced daily variation. 3 are as follows :

3.2. Connections between mid- and high-latitude pulsations

Connections between the daily average Pc3 ampli- tudes of the pulsations are shown in Fig. 2, for the pairs HSD-TOS, NYA-NCK and TOS-NCK. As for normalization, daily average amplitudes are expressed in units of the corresponding monthly average ampli- tudes. The (orthogonally least-squares fitted) con- nections between these daily average amplitudes and the scatter of the individual values are given in Table

(1) The scatter at individual stations is similar with the exception of the aurora1 zone station TOS, where

it is higher (60% instead of 40% elsewhere). (2) At very high-latitude stations there is a part of

the pulsation activity which is independent of the lower-latitude activity. This ‘remanent activity’ is pre- sent even if there is no pulsation activity at lower latitudes, and in such a case it is 2430% of the average pulsation activity (intercepts in Fig. 2 and numerical values in Table 3). At the aurora1 zone station TOS,

192 J. VERB et al.

R=+O 85

TOS

NCK

20

0 05 I .o 1.5 2.0 2.5 0 0.5 1.0 I.5 2.0 0 0.5 I .o 1.5 2.0 2.5

WD NYA TOS

Fig. 2. Dependence between daily average amplitudes of Pc3 at different stations with least-squares regression lines (in an orthogonal sense, i.e. the sums of the squares of the distances between points and the regression line is a minimum). Correlation coefficients (R) are also shown. All values are referred to

the average.

however, no such ‘remanent activity’ exists (more

exactly, it is 2% of the average activity).

The ‘remanent activity’ is either continuously pre-

sent at very high latitudes, or it changes independently of the lower-latitude activity. This additional high-

latitude pulsation activity should have a source being

independent of the pulsations elsewhere. The aurora1 zone pulsation activity, however, has no such com-

ponent in spite of being more variable than both at

lower and at higher latitudes. In spite of these differences in detail, longer time,

e.g. daily, averages of the pulsation activity are rather

highly correlated (see Table 3, correlation coefficients

between 0.66 and 0.85), even between very high- and mid-latitude stations.

Figure 3 shows a close connection between HSD Pc3 and NCK Pc3 (Pc3 is meant here in a somewhat

restricted sense, with an upper limit of 30 s; see VER& 1980) and Pc5, while NCK Pc2 and Pc4 are uncor-

related. Such a close connection between the two Pc3 activities may indicate their common origin at the two

localities. Pc2 and Pc4, however, should have some

other kind of excitation mechanism at NCK, as they

do not depend on HSD Pc3 activity. As at very high latitudes, Pc4 amplitudes have a

similar daily variation to those of Pc3 ; a comparison

was also made between NYA Pc4 amplitudes and NCK Pc3 amplitudes. Figure 4 shows that the con-

nection is, especially in the case of small NYA ampli- tudes, nearly linear with a correlation factor of +0.8.

At higher activities, the scatter increases (at least partly due to a lower number of cases). At NYA, Pc5 is also correlated with mid-latitude Pc3, at least for

quiet and moderately disturbed conditions. Figure 5

shows this connection for NYA Pc5 and NCK Pc3. If the NYA Pc5 amplitude is less than average, NCK

Pc3 amplitudes are roughly proportional to them; for greater NYA Pc5 amplitudes, the connection is continuously deteriorating. This close connection

backs up the previous conclusion that, at higher lati-

tudes, Pc4 and even Pc5 (at least in low amplitude events) may belong to the same (physical) class of pulsations as Pc3 at lower and higher latitudes.

Table 3

Scatter of the daily average amplitude

at the stations Best fitting line Correlation

coefficient

HSD + 0.45 TOSi0.6

A HsD = ATOS x 0.70+0.30 0.85

NYAk0.4 NCK ,0.4

A - A,,, NYA - x0.75+0.24 0.66

TOSk0.6 NCK + 0.4

A - A,,, TOS - x 0.98 + 0.02 0.57

Connections between high- and middle-latitude pufsations 793

--PPCZ- JJNC,

-Pc3- PC4 ----Pc5-

3

i

l . .

NCK 5-15s 15-30s I

30-60s . I-2min 2-IOmin 2

l . . . .

. . . .

I-. 0. . .

. . . l . .

I I I I I 1 I I I I I I I I I I I t I I I 0 12340123401234012340~234

AHSO ,Pc3

Fig. 3. NCK amplitudes in several period ranges vs HSD Pc3 amplitudes. Close connections are in the

ranges 15-30s and 2%i0min.

ro-

8- 0

> IIIIIII II i I I I

0 I23456 - 910 - 1415- 19x,

ANCK, PC3

Fig. 4. Connection between low-latitude (NCK) Pc4 and high-latitude (NYA) Pc3 amplitudes. Averages for hourly

average NCK Pc3 amplitudes are shown.

N-89N=144 N=74 N-17 N=73N=25 N-32 NCK PC3 NCK PC5

WA PC5 Relative units

Fig. 5. Connection of NYA Pc5 amplitudes with NCK Pc3 and Pc5 amplitudes. Number of cases (N) is given on top of

the figure.

Table 4.

Hours Regularity index Share of regular

n-l n+l -53 -n30 -10

waveforms (0, Q) 16 23 31 (% at NCK)

The regularity of the pulsation events is con- tinuously determined at NCK based on a subjective estimation of the scatter of the periods and on the admixture of periods differing from the main peak within each time interval. The regularity is char- acterized by the following four letters: 0 means a period scatter of C lo%, Q+50%, W+ lOO%, and T represents irregular pulsations. The more regular the pulsations are, the more active field line resonances are (and, correspondingly, the period of the pulsations changes at mid-latitudes more quickly in O-class events than in any other class, and in the W- and T- classes, latitude-dependent events are uncommon). In spite of the visual determination, the regularity index (2 x 0 + Q -T, where 0, Q, W and T are expressed in per cent of all cases) reflect quite well the changes in field line resonance effectiveness.

In a superposed epoch analysis, n, the key hour of the events was 1 h when the Pc3 amplitude in HSD surpassed 4 units. A total of 72 such events were found, and the regularity of the corresponding NCK Pc3 pulsations changed as shown in Table 4. The regularity in NCK increased significantly, if the Pc3 activity increased at HSD.

In Fig. 6 the regularity indices for NCK Pc3 (per- iods less than 30s) are plotted vs Pc3 amplitudes at TOS. The regularity of Pr3 increased very strongly at NCK vs TOS amplitudes in both months studied.

These results can be summarized as follows : in the case of pulsations of magnetospheric origin, the field

Regu(arlty index

I-100

I- 60 l T(30s

-60

\ .

-40 ’ .

-20 . l

0 \

.

100 .

J. VERB et al.

011 0 I 2 3 4 5u6>6

ATOS

Fig. 6. Regularity of the Pc3 pulsation at NCK vs Pc3 amplitudes at TOS, for periods below 30 s. Ton : reeularitv index increasing downwards (for explanation, see tex;). Bat- tom: occurrence of different types of regularity (also

explained in text).

line resonances create very regular pulsations at mid-

latitudes, while in the amoral zone the amplitudes increase very strongly, the spectrum gets complex, and additional peaks appear. This situation is described, for example, by Cz. MILETITS et al. (1990). Pulsations of solar wind origin, however, are less regular at mid-

latitudes ; the spectrum for such events is sometimes quite complex there, while in the aurora1 zone they

have less amplification (therefore they can be seen mostly in quiet times). Thus smaller amplitude aurora1

zone events have less regular mid-latitude counter- parts (Cz. MILETITS, 1989).

4. CONCLUSIONS AND DISCUSSION

The present investigations have shown statistically that the mid-latitude (NCK) Pc3 activity is closely correlated with high-latitude Pc3, both in the aurora1

zone (TOS) and further toward the pole (HSD and NYA). Moreover, high-latitude Pc4 and 5 activities are also correlated with low-latitude Pc3, with the difference between the two stations being that, at HSD, a period change did occur according to WARNER and ORR (1979) and SINGER et al. (1981) in the morning hours, while at NYA this is not the case. It is thus probable that there is a rather narrow zone,

between latitudes of about 68 and 73 , where the daily

change of the toroidal eigenperiod can be observed at

the Earth’s surface. Equatorwards of this zone, the

change is too small to be detected; to the north of it,

in the vicinity of the cleft/cusp position, the morning shortening of the period is no more observable. Thus the local time control of the pulsation periods is com- plex at these latitudes. It should not be forgotten

that these results refer to geomagnetically quiet or moderately disturbed times.

Morphologically, four different groups can be dis-

tinguished in the Pc3-5 pulsation activity.

(1) Pulsations of the solar wind origin type. The characteristic period is 20-30 s, i.e. it is within the Pc3

range. The period in seconds corresponds to 160/B, where B is the scalar magnitude of the interplanetary

magnetic field, in nT. Thus typical values of B here are 5510 nT. The period does not change with geo-

magnetic latitude, and the activity of this type includes the polar cap region, too, at least during quiet and very quiet times. It is characteristic that for a sample

selected from the present material (regular pulsations at TOS), no latitude-dependent event was found in a detailed study using dynamic spectra (Cz. MILETITS,

1989) ; thus all such events belonged to this type. Type 1 was studied among others by ENGEBRETSON

et al. (1986), and they also found a close correlation

with the cone angle. Thus this type shows all the characteristics of the upstream wave (see also WOLFE et al., 1987; YUMOTO et al., 1987). ENGEBRETSON et

ul. (1987) found arguments that compressional waves

(of solar wind origin) penetrate from high latitudes toward lower latitudes ; nevertheless, equatorial entry is considered likely. This would explain the constant

period across a wide range of latitudes and an ampli- tude maximum near the cusp/cleft position.

(2) Pulsations of inframagnetospheric origin, The

period changes from about 20s at mid-latitudes (i.e

from the period of the solar wind origin type) to several minutes at high latitudes, and even with the time of the day (local time). The activities of Types 1 and 2 often appear simultaneously ; sometimes, how-

ever, they appear separately. Several cases were found when low-latitude Pc3 coincided in time with high- latitude Pc4 and 5 (Cz. MILETITS~~ al., 1990). YUMOTO

et al. (1987) consider that the bulk of the high-latitude pulsation activity is of K-H origin. The K-H insta- bility may excite field line resonances as upstream waves do (SOUTHWOOD, 1974 ; CHEN and HASEGAWA, 1974) but the former are restricted to higher latitudes. Such a generation fits WEDEKEN’S (1984) observation of changing periods in the PC&5 range at Scan- dinavian stations. It is, however, unclear how the lati-

Connections between high- and middle-latitude pulsations 795

tude-dependent periods change smoothly across the plasmapause. This smooth change was attributed to

resonances in detached plasma regions outside the

plasmapause, but the occurrence of such cases is too often for this to be the explanation. Another possi- bility is that cold heavy ions exist in greater con- centrations outside the plasmapause, as suggested by FRASER et al. (1988). Nevertheless, the fact that small- scale structure of the simultaneous low-latitude Pc3

and the high-latitude Pc4-5 do not coincide speaks in favour of a different origin, controlled by a common

factor (the solar wind velocity). It is characteristic that high-latitude Pc3 and Pc4

pulsations were found to be highly correlated both by YUM~T~ et ul. (1987) and in the present study. Type

2 has thus a broadband character, with enhancement

al the local field line resonant period, and it does occur at somewhat disturbed times, too.

(3) Low-latitude Pc4 pulsations. These pulsations

may be of mixed origin : partly the spectral peak cor- responds to the period expected on the basis of the IMF scalar magnitude, and partly no such cor- respondence is found. Such Pc4 pulsations have been

identified several times, for example by GUL’ELMI

(1974). The waveform is in both cases rather irregular

due to the lack of field line resonances; the period does not change with latitude. Such waves are absent at high latitudes (or they have too small an amplitude to be detected in a more noisy background) and at

high geomagnetic activities (may be due to the same cause as at high latitudes). Low-latitude Pc4 does

not transfrom into high-latitude Pc3. YUMOTO et al. (1987) have also shown the independence of Type 3 (low-latitude Pc4) activity from Pc3.

(4) High-latitude Pc3. About one-quarter of the high-latitude Pc3 activity is independent of low-lati- tude Pc3 (present study). The morphology of this type is unknown. It seems probable that it may be the high-

latitude part of the broadband activity (Type 2), being

uncorrelated with low-latitude activity. If the (upstream) source of the pulsations has a

spectrum with a peak at a period (in s) of 160/B, with B in nT. and a rather sharp cut-off toward shorter

periods and a wide tail toward long periods, then Type 1 could correspond to the peak of the primary spectrum (at 160/B), with the wide tail of local field line resonances enhancing signals with periods which

increase with geomagnetic latitude. Type 1 has. more- over, amplification in the polar cap (PLYASOVA-BAK-

OUNINA et cd., 1986), and Type 2 at much lower

latitudes. Type 3 may correspond to low B-values, when at low latitudes no field line resonances are

possible, but impulses embedded in the primary field

may excite, notwithstanding the local resonant period (VER& 1980). This is most typically the case at high

latitudes, or also for signals coming from processes at the plasmapause. Type 4 is only present at high latitudes.

CHEN L. and HASEGAWA A Cz. MILETITS J.

Cz. MILETITS J., VERBS J., SZENDR~I J., IVANOVA P., BEST A. and KIVINEN M.

EGELAND A., DEEHR G. S., SIVJEE G. G., HENRIKSEN K. and SMITH R. W.

ENGEBRETSON M. J., MENG C-I., ARNOLDY R. L. and CAHILL L. J

ENGEBRETSON M. J., ZANETTI L. J., POTEMKA T. A., BAUMJOHANN W., LI~HR H. and ACLJNA M. H.

FRASER B. J., MCPHERRRON R. L. and RUSSELL C. 7 GUL’ELMI A. V. NEWELL P. T. and MENG C-I. ORR D. and MATTHEW J. D. A. PLYASOVA-BAKOUNINA T. A.. STUART W. F.

and CHARCHENKO L. P. PLYASOVA-BAKOUNINA T. A., TROITSKAYA V. A.,

MUNCH J. W. and GAULER H. F. SINGER H. J., SOUTHWARD D. J., WALKER R. J. and

KIVELSON M. G. SOUTHWOOD D. J. VERA J. WAKNER M. R. and ORR D. WEDEKEN U.

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